Plato’s Forms are not the same as Kant’s things-in-themselves. Nevertheless, they stand in an equivalent position to the world of direct experience: that is, they symbolize something about what might lie beyond direct experience.
We cannot function in ordinary life after a certain point without some awareness that a world lies beyond direct experience. The infant cries in part to summon help from a person no longer present. The student opens the school house door on the assumption that a classroom waits on the other side. I infer the existence of a heart in my own chest, based in part on what I have seen in diagrams from textbooks, but also as a logical inference about the way my blood circulates. Something does that work. Besides, I can actually hear it, even if I cannot see it.
In everyday experience, we encounter physical surfaces like a drapery over the rest of the world. We touch an orange rind and presuppose the fruit inside. We pick up a book in order to read something in its pages. Somebody leaves the room, and we do not assume she immediately ceases to exist. In fact, our folk wisdom is often based on the perils of making inferences about the world beyond direct experience, inasmuch as “you can’t judge a book by its cover” and “all that glitters is not gold” and “look before you leap”.
I recall there was once a warning for automobile drivers not to assume that a paper sack in the road is empty. Driving over it is also driving over its contents, such as a broken bottle that might puncture your tires.
Scientists have taken the distinction between surfaces and depth to heart and resolved to penetrate surfaces, to find out how it all really works. On a map of the globe from the 1500’s, an unexplored region bore this inscription, “Here there be monsters.” The universe presents the curious with boundless opportunity to probe and peer – to cut open the skin, to dive into the sea, to watch the microbes dance, and to witness stars being born. And with the accumulation of knowledge, humans win an increasingly rich understanding of a reality that otherwise eludes our native powers.